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> I would like to achieve is 0.5 sec over 8 hours or roughly 17 ppm
> over a temperature range of -20 C to +40 C

If you assume, not unreasonably, that the chip out of a "kitchen"
clock is good to ~ 1 minute/year, that's around 2ppm. I use them
as the timebase in many projects. You won't find better performance
cheaper

Attached is a snippet from an old application. '1' and '2' are where
the coil used to attach. These are reciprocating lines, ie 0.5Hz AC,
to reverse the polarity of the coil each second (kitchen clocks are
magnetic drive). Take one for 0.5Hz or both for 1Hz

Nominal supply is a 1.5V AA, and they use only a few 10's of nA.
I've not tried a temperature range test but that should be easy to
do. Synchronise any two clocks and put the one you want to pinch
the chip out of in the freezer

If there is a significant difference I'm sure that temperature logging
could be used for compensation. Either that or provide a small
local heater - eg resistor - for the chip or whatever oscillator you
eventually use

Joe=

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